Chiavennite (Mn endmember) and ferrochiavennite (Fe endmember) are beryllium silicate zeolites with chemical compositions close to Ca1–2(Mn,Fe)Si5Be2O13(OH)2·2H2O. Five chiavennite crystals from four different localities, Chiavenna, Italy; Telemark, Norway; Utö, Sweden; and Ambatovita, Madagascar, were chosen for crystal-structure analysis. Initially all had been reported to be orthorhombic, but all are monoclinic, with a β angle near 90°, as is ferrochiavennite from Blåfjell, Norway. Twinning on {100} is common. Chiavennite and ferrochiavennite are isostructural. The zeolite structure consists of intersecting channels of nine-, six-, five-, and four-fold rings. All five structures have one Ca site in common while chiavennite from Utö and ferrochiavennite from Blåfjell have a second, partially filled, Ca site. Hydrogen atoms were located in all five structures. Hydrogen bonding is similar but not identical in all five structures. The H atoms associated with BeO4 tetrahedra act as terminations to a polymerization sequence causing chiavennite and ferrochiavennite to be interrupted zeolite structures.

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